1. Technical Field
The invention relates to induction melting and heating applications such as electrical furnaces and in particular to a composite insulated clamp assembly for securing selected windings of an induction coil in a fixed position.
2. Background Information
The windings of an induction coil of frequency melt equipment, and in particular the terminal or end winding must be maintained in a secured position at the start and end of the coil to prevent unwanted expansion and movement of the coil with respect to the material receiving vessel. A tie-bolt assembly is presently used today on many types of induction furnaces for this purpose, but possesses a number of possible inherent problems. These tie-bolt assemblies are fixed to the top and bottom windings or turns in the coil and are usually located directly adjacent to and in contact with the poured cast rings and the backside of the refractory lining in the free board areas of the furnace as well as the bottom cast ring beneath the coil that supports the working furnace lining. These locations of the tie-bolt assembly subject the insulators at the top and bottom sides of the coils to a direct thermal transfer from the hot cast rings as well as from the back side of the furnace liner. The tie-bolt assembly is secured to the second winding of the coil with a tie-in lug which is usually a large mass of copper welded to the top of the second winding of the coil. This copper mass is far from the coolant path of the coil and is extremely thermally conductive and will soak up heat from the cast ring and lining contributing to the total heat load of the tie-bolt assembly. These tie-bolts are supported on insulators which are subjected to thermal degradation and can lead to burning or charring and create an arc over from the tie-bolt resulting-n tie-bolt failure.
Another problem with such tie-bolt assemblies is that they are subjected to radial stress on the insulator since the bottommost turn of the coil is subjected to the greatest degree of head pressure from the molten bath and consequently, the highest degree of radial stress. This can fracture and split the insulator supporting the tie-bolt assembly or cause cracks which subjects the insulator to possible arcing.
Still another problem that exists with such insulated supported tie-bolts occurs form radial flux. The position of the standard tie-bolt assembly, and in particular the copper lug which is welded to the second winding or turn at each end of the coil, places each lug behind a magnetic yoke which puts a large mass of copper directly in the path of the concentrated radial flux lines entering the shunt. This causes the copper block or lug to become inductively heated which exceeds the permissible temperature on the insulator, which combined with the thermal transfer from the hot cast rings of the furnace, contributes to the fracturing or cracking of the insulator.
The present invention provides a clamp assembly for the coil of an induction furnace which avoids the thermal degradation, radial stress and radial flux experienced by prior art end winding terminations, such as the tie-bolt assembly commonly used on many types of induction coils.
The clamp assembly of the invention comprises a T-shaped latch bar which is formed of a high temperature, high strength, high dielectric, high pressure, glass reinforced laminate insulating material which has one or more holes in the stem portion of the T-shaped bar for securing the bar to one of the windings of the induction coil.
Another aspect of the invention provides a pair of studs formed of relatively small masses of copper or other metal, welded to coil windings adjacent to the winding along which the stem of the latch bar extends. These lugs are received in notches formed in the cross member of the T-shaped latch bar and absorb the radial stress loads and strain on an adjacent power connection.
A further feature of the invention is the placement of the T-shaped latch bar on the face of three adjacent windings where it is kept relatively cool by the cooling water which flows through the interior of the induction coil windings.
Another aspect of the invention is the location of the T-bar on the outer face of the coil which isolates it from the cast rings or furnace linings, thereby avoiding exposure to the heat generated thereon, and since the two securing lugs are located on the face of two spaced windings they are not subjected to radial flux and thus will not inductively heat and cause thermal degradation of the T-shaped insulated latch bar.
Still another important aspect of the present invention is that the T-shaped latch bar can be used with many types of induction furnaces requiring only minor modifications thereto thereby enabling it to be used on new coils or retrofitted onto existing coils.
The foregoing advantages, construction and operation of the present invention will become more readily apparent from the following description and accompanying drawings.